The present invention relates to an arrangement for correcting or compensating for magnetic declination in an optically readable bearing compass whose body encloses a hermetically sealed capsule containing a wind rose compass card and is provided with a main reading line calibrated to indicate bearing to the earth's magnetic pole without correction of declination. The body of the bearing compass has an opening equipped with an optical system suited to facilitate taking the bearing indicated by the compass card.
In a customary construction of conventional bearing compass, the compass is equipped with a compass card, e.g., a wind-rose-type compass card, and one reading point or reading line visible through the optics of the compass. Correction of declination, also called variation, cannot be arranged in this type of compass. In this note, the term "variation" generally connotes the deviation of the magnetic needle of the indicator from the true north direction which varies from place to place and is determined by the direction of the horizontal component of the earth's magnetic field which is called declination.
Correction of declination is extremely important as the difference in degrees on a 0-360.degree. scale between the bearing to the magnetic north pole and the bearing to the geographic north pole, or true North Pole in many areas of the earth may amount up to about 30.degree. and above. Due to declination, maps are printed marked with the local values of declination. As an example it may be mentioned that, depending on the area, declination in the North-American continent varies from 35.degree. eastern declination to 40.degree. western declination.
In conventional bearing compass constructions with a wind rose, the correction for declination in order to obtain the true bearing has usually been possible only by first reading the normal magnetic bearing and then either adding or subtracting thereof the local value of declination taken from the map, depending on whether the declination is eastern or western declination. Hence, the acquisition of true bearing has always necessitated calculation with basically simple rules, which in some cases are even misinterpreted so that the value of declination has been added to the bearing reading while it should have been subtracted. Thus, a double error has been introduced into the bearing measurement.